This invention relates to nuclear magnetic resonace (NMR) apparatus. More specifically, this invention relates to radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals.
In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate test-tube samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in both humans and animals. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.o, a greater number of nuclear-magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.o, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the Larmor equation .omega.=.gamma.B, in which .gamma. is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B.sub.o plus other fields) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.o, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonal to the direction of magnetization M by means of radio-frequency pulses through a coil connected to radio-frequency-transmitting apparatus. Magnetization M rotates about the direction of the B.sub.1 field. In NMR, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B.sub.o field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane begin to realign with the B.sub.o field by a variety of physical processes. During this realignment process, the nuclear moments emit radio-frequency signals, termed the NMR signals, which are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. The same or a second RF coil may be used to receive the signals emitted from the nuclei. In NMR imaging applications, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the NMR signal. This information is later used to reconstruct images of the object studied in a manner well known to those skilled in the art.
In performing NMR studies, it has been found advantageous to increase the strength of the homogeneous magnetic field B.sub.o. This is desirable in the case of proton imaging to improve the signal-to-noise ratio of the NMR signals. In the case of spectroscopy, however, this is a necessity, since some of the chemical species studied (e.g., phosphorus and carbon) are relatively scarce in the body, so that a high magnetic field is necessary in order to detect usable signals. As is evident from the Larmor equation, the increase in magnetic field B is accompanied by a corresponding increase in the resonant frequency of the transmitter and receiver coils. This complicates the design of RF coils which are large enough to accommodate large objects such as the human body. One source of difficulty is that the RF field produced by the coil must be homogeneous over the region to be studied. Another complication arises from the intrinsic distributed inductance and capacitance in such large coils which limit the highest frequency at which the coil can be made to resonate.
Presently used coils employ one turn or two turns in parallel to minimize the inductance and increase the resonant frequency. The concentration of the resonant current in so few turns reduces the homogeneity of the B.sub.1 field, as well as the homogeneity of the sensitivity to signals produced in different parts of the sample region. Moreover, the lack of symmetry between the position of the tuning capacitor and the stray capacitance of the single-turn coil lead to a non-uniform current distribution in the coil and a corresponding reduction in the uniformity of the B.sub.1 field and signal sensitivity.
It is, therefore, an object of the invention to provide an RF coil capable of generating a substantially homogeneous B.sub.1 field and which has substantially uniform signal sensitivity over the region of interest.
It is another object of the invention to provide an NMR RF coil which is operable at lower RF power and which exhibits an improved signal-to-noise ratio.
It is still another object of the invention to provide an NMR RF coil having current and tuning capacitance distributed in many turns but which has an effective inductance of a single turn.